1. Field of the Invention
The present invention relates to an object recognition method and apparatus for determining the presence and shape of an object by means of image processing of a video signal obtained with an imaging apparatus such as a television camera.
2. Description of Prior Art
Determining the presence and shape of an object by means of image processing has become an important factor in the automation of production processes in recent years.
One conventional recognition method as disclosed in the Japanese Patent Publication No. 52-41016 (issued in 1977) is described below with reference to FIG. 1. FIG. 1 shows a recognition object image 1 being processed with a mask plane or mask image 2 having a plurality of partial areas 3a-3g at specific portions. The recognition object image 1 is an image obtained by imaging a seven-segment display apparatus. In this case, light emitting segments of the display apparatus are treated as a recognition object 5 contained in the recognition object image 1.
Areas where the partial areas 3a-3g overlap the recognition object 5 are regarded as presence areas 6a-6g.
According to the conventional method, the first step in the recognition process is to separate the presence area 6a from the background B within the partial area 3a by some known technique. Then, the area of the presence area 6a in the partial area 3a is obtained. A variety of methods are used to separate the presence area 6a from the background, but the most common is a binary digitization process whereby a value of 1 is assigned to densities greater than some predetermined threshold value, and a value of 0 is assigned to any densities lower than that threshold value.
If the presence area 3a so separated has a brightness which is larger than a predetermined value, it is determined that .at least one part of the object 5 is present. Specifically, it is determined that the highest of the seven segments is illuminated. This same presence evaluation is thereafter repeated for the presence areas 6b-6g in the other partial areas 3b-3g. If the presence of the object 5 is confirmed in the partial areas 3a-3d and 3g as a result of the above object presence determination operation, then a prepared truth table can be referenced to recognize that the 7-segment display is showing the number "3".
As described above, this method requires that a predetermined threshold value be set to separate each presence area 6a-6g from the background. However, it is difficult to determine such a threshold value that can reliably separate the presence area of the object from the background unless the contrast of the image 1 is good. An image on a liquid crystal display can be cited as an example of an image which does not have good contrast. In addition to the above disadvantage, an additional means for setting a threshold value must also be provided.